Cryogenic process



1970 E. H. VAN BAUSH 3,546,?

CRYOGENIC PROCESS Filed March 12, I 1968 T0 DISTRIBUTION PRODUCT EDWARD H VAN BAUSH INVI'JNI'()H.

United States Patent 3,546,892 CRYOGENIC PROCESS Edward H. Van Baush, Pearl River, N.Y., assignor to Hydrocarbon Research, Inc., New York, N.Y., a corporation of New Jersey Filed Mar. 12, 1968, Ser. No. 712,532 Int. Cl. F25j 3/02, 3/04, 5/00 US. Cl. 6213 3 Claims ABSTRACT OF THE DISCLOSURE A process for increased liquid production from a cryo BACKGROUND OF THE INVENTION It is normal practice in air separation plants to have a fractionation step which may consist either of a double fractionation tower, a single high pressure fractionation tower with a reboiler on top, separate high and low pressure fractionation towers, etc., to remove a nitrogen overhead vapor stream, usually from the high pressure fractionation tower and to use this stream to cool the incoming air. This stream, thus warmed, is at a temperature intermediate between ambient and the minimum process temperature and is then expanded to supply refrigeration which can be used in the process. Normally, in processes involving the use of a high and low pressure fractionation tower, in addition to the nitrogen overhead vapor stream, liquid nitrogen reflux and liquid-rich air streams are removed from the high pressure tower, passed in heat exchange with a cold low pressure vapor stream from the low pressure tower and throttled into the low pressure tower. The cold low pressure vapor stream is then used to cool the incoming air.

Normally, the refrigeration obtained from the expansion of the high pressure vapor stream is used to make the liquid product streams. When it is desired to increase the amount of these liquid streams, particularly in the case of liquid oxygen, the flow to the expansion step must be increased to obtain increased refrigeration". This means that an increased flow of vapor must be removed or bypassed from the high pressure distillation tower.

Such increased flow has several adverse effects on the system. First, as the flow increases, the temperature level of the expansion decreases and the refrigeration output per unit flow of the expansion is lowered. Second, it becomes necessary, in order to compensate for the loss in refrigeration output, to increase the flow through the expander which tends to reduce the temperature even further. As the higher flow depletes the vapors in the high pressure tower, the ability of the fractionation step to produce liquid is reduced and, consequently, less liquid reflux is available to absorb the refrigeration produced.

Thus, in spite of the fact that enlarging the flow through the expansion step gives increased refrigeration, the amount of such refrigeration and resultant liquid production is quite limited.

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SUMMARY OF THE INVENTION I have discovered a method by which increased liquid production may be achieved in an air plant by using increased fiow through the expansion step without suffering the deleterious effects of such an increased expander flow as described above.

My invention is particularly useful in a plant wherein a high pressure gas stream is removed as product. The nature of this stream is not critical to the invention, but it would usually be oxygen or nitrogen. A typical case would be a plant which produces vapor oxygen which is stored on the site at high pressures. If full vapor production is not required, this supply may be used as the compressed gaseous stream for my invention. If increased liquid production is required, it is only necessary that this stream be at a pressure sufiicient such that it will condense against the sensible heat of the high pressure vapor stream.

Specifically, I have found that by taking a portion of the nitrogen overhead vapor from the high pressure fractionation tower, prior to its being expanded, and passing it in heat exchange with the compressed stream described above, one may liquefy a substantial amount, if not all, of the compressed stream. The warmed nitrogen overhead vapor stream from said heat exchange is then recombined with the remainder of the nitrogen overhead vapor which has been passed in heat exchange with the air feed. The recombined stream is then expanded in the usual manner to give refrigeration which is then added to the system.

It is seen that the additional heat exchange of this portion of the nitrogen overhead vapor will require the removal of additional vapor from the high pressure tower, i.e., increased flow through the expander. Since, however, this heat exchange is essentially external of the system, the temperature level of the expansion remains the same as it would be had a lesser flow been used without the invention. Particularly, it has been found that the re frigeration requirement for the liquefaction step can be matched by the refrigeration produced across the expansion step, such that with a specified amount of liquid being produced, the amount of liquid obtained from the fractionation steps will not be substantially decreased. Also, because of the higher temperature level of the expansion step, a relatively high refrigeration output per unit fiow is obtained from across the expander.

If the high pressure stream is a high pressure oxygen vapor product from the system, a substantial increase in the amount of liquid oxygen from the process may be obtained without significant reduction in the amount of liquid nitrogen reflux and rich air streams.

Thus, it is seen that by the use of my invention, increased liquid production may be obtained from air plants while utilizing increased flow through the warm expansion step without substantial lowering of the temperature of the expansion step and without substantial lowering of the normal liquid output of the process fractionation step.

DESCRIPTION OF THE DRAWING The drawing is a schematic flow diagram of an air separation plant.

PREFERRED EMBODIMENT OF THE INVENTION As shown in the drawing, air feed at 10 is compressed to about p.s.i.a. in compressor 12 and is then cooled to about 275 F. by passing through reversing heat exchangers 14 and 15. The air stream is saturated at this temperature. It then is introduced through line 16 to the high pressure fractionation tower 18, where it is fractionated into a nitrogen overhead and a rich air liquid containing about 38% oxygen. The pressure in this tower varies from about 87 p.s.i.a. at the bottom to about 85 p.s.i.a. at the top.

The rich air liquid is removed in line 32, passed into heat exchanger 22 where it picks up refrigeration after which it is expanded through reducing valve 36 and introduced to the low pressure fractionation tower 28 through line 34.

The nitrogen overhead is split into a high pressure nitrogen stream and a nitrogen reflux liquid. The reflux liquid is removed from tower 18 through line 20, is passed through heat exchanger 22 where it also picks up refrigeration and then leaves through line 24, is expanded through reducing valve 26 and is introduced into the low pressure fractionation tower 28. The pressure in tower 28 varies from about 22 p.s.i.a. at the bottom to about 19 p.s.i.a. at the top. Most of the vapor in the high pressure tower 18 is condensed in reboiler 30, the heat from the condensation being utilized in the low pressure fractionation tower 28. A cold effluent stream is removed from the low pressure tower in line 42, used to cool the nitrogen reflux liquid and the rich air liquid from the high pressure tower in exchanger 22, and is then passed to reversing exchangers and 14 where it is used to cool the incoming air feed and discharging at 44.

A high pressure nitrogen vapor stream is removed from the high pressure tower 18 in line 41, said vapor being saturated and at a temperature of about -285 F. A first portion of this stream from line 41 is used to cool the air feed in reversing exchanger 15 and exists reversing exchanger 15 through line 45 at a temperature of about 120 F. Some of the high pressure vapor stream in line 41 may bypass reversing exchanger 15 through line 50 and valve 52. A second portion of the high pressure vapor stream in line 41 is removed through line 48 and passed through heat exchanger 54, exiting heat exchanger 54 through line 56 at a temperature of about 226 F. This second portion of the high pressure vapor stream in line 56 is then recombined with the first portion of the high pressure vapor stream in line 46. The temperature of this recombined stream is about -l90 F. It is expanded in expander 58 and exits at a temperature of about 280 F. It is then introduced through line 59 into the cold efiluent gas stream whereby the refrigeration gained from the expansion step in expander 58, is added to the system.

An oxygen product gas is removed from low pressure tower 28 through line 38, is passed through reversing exchangers 15 and 14- and is removed in line 60 at a temperature of about +95 F. It is then compressed in compressor 62 to the required distribution pressure. After removal of the heat of compression in cooler 63, all or a portion of the product gas may be bypassed through line 66 to distribution. However, when liquid is required in place of high pressure gas, all or a portion of this high pressure gas may be passed through line 64 and through heat exchanger 54 wherein it would be cooled to about 280 F. This would result in a substantial portion or all of the oxygen becoming liquid so that liquid product, in addition to that normally produced, could be obtained from the plant. It is not necessary, of course, that the high pressure gas stream liquefied in exchanger 54 be oxygen, but it may be any product vapor stream from the plant which it is convenient to use. It could also, of course, be a gas stream such as nitrogen extraneous to the plant added through line 61. In such case, oxygen would be removed in line 60a rather than entering compressor 62 and liquid nitrogen would be produced at 69.

The partially liquefied stream in line 68 may be removed as product through line 69 or may be throttled into the low pressure tower 28 through valve 72 and line 71. This allows recovery of the vapor in this stream and liquid may then be removed as product from the tower 28 through line 70. The separation of the vapor and liquid as described may alternately be carried out in a vessel external to the tower.

It is noted that it the refrigeration required for the high pressure gas product in line 64 is matched with the refrigeration duty realized from the expansion in expander 58, large flows may be expanded without a detrimental effect on the overall air plant operation, except for reduction in the overall oxygen recovery. Since the liquefying of the compressed gas product stream is accomplished at a temperature level higher than 285 F., and this level of refrigeration, in turn, is recovered when the expander takes the warm unbalance stream and discharges it back to the system at about --280 F., there is no refrigeration recovery imposed on the reflux streams in lines 20 and 32, which would otherwise be sub-cooled to recover the refrigeration from the expander.

Since the compressed gas product stream is an oxygen stream as described above, and the liquid oxygen so produced is returned to the main tower, then the equivalent liquid, such as nitrogen in line 73 and argon could be recovered in place of oxygen. Thus, it also follows that any high pressure stream could be utilized as the liquefied stream if it does not have to be returned to the reversing exchangers.

Since the expansion conditions remain in the warmer range of temperatures, the flow taken from the high pressure tower remains at a minimum which, in turn, results in relatively high oxygen recovery. In addition, the magnitude of this flow through the expander can be increased to relatively high proportions of the total air feed to the plant without upset of the reversing exchanger temperatures or mass balance.

Inasmuch as the description above discloses a preferred mode of embodiment of my invention, it is recognized that from such disclosure, modifications within the spirit wll be obvious to those skilled in the art and it is understood, therefore, that my invention is not limited to only those specific method steps, or combination or sequence of method steps described, but cover all equivalent steps or methods that may fall within the scope of the appended claims.

I claim:

1. In combination with a cryogenic process for the separation and liquefaction of gaseous components of the atmosphere of the type wherein air is compressed, cooled by passing in heat exchange with cold eflluent gases and then fractionated and wherein a high pressure nitrogen stream is removed from the fractionation step, warmed to a temperature substantially greater than the minimum temperature of the process, but substantially lower than ambient, and then expanded and used to supply refrigeration to the process, the improvement which comprises passing a first portion of the high pressure nitrogen stream in heat exchange with the incoming air feed and passing a second portion of the high pressure nitrogen stream in heat exchange with a cold compressed gaseous stream and then recombining the second portion with all other portions of the high pressure vapor stream prior to expansion said compressed gaseous stream being a vapor nitrogen product stream obtained from the process and liquid nitrogen is produced.

2. In combination with a cryogenic process for the separation and liquefaction of gaseous components of the atmosphere of the type wherein air is compressed, cooled by passing in heat exchange with cold effluent gases and then fractionated and wherein a high pressure nitrogen setream is removed from the fractionation step, warmed to a temperature substantially greater than the minimum temperature of the process, but substantially lower than ambient, and then expanded and used to supply refrigeration to the process, the improvement which comprises passing a first portion of the high pressure nitrogen stream in heat exchange with the incoming air feed and passing a second portion of the high pressure nitrogen stream in heat exchange with a cold compressed gaseous stream and then recombining the second portion with all other portions of the high pressure vapor stream prior to expansion said compressed gaseous stream being a vapor oxygen product stream obtained from the process and liquid oxygen is produced.

3. The process as claimed in claim 2 wherein the liquid oxygen is returned to the fractionation step and liquid products other than oxygen are recovered from the process.

References Cited UNITED STATES PATENTS 2,513,306 7/1950 Garbo 62--12 6 Scharmann 6213 Schilling 6229 'Seidel 6239 Peruier 62-39 Cimler et a1 62-29 NORMAN YUDKOFF, Primary Examiner A. F. PURCELL, Assistant Examiner 2/1952 Garbo 62-13 10 6230, 39

US. Cl. X.R. 

